Once more unto the bray

July 23rd, 2008 by gavin

We are a little late totheparty, but it is worth adding a few words now that our favourite amateur contrarian is at it again. As many already know, the Forum on Physics and Society (an un-peer-reviewed newsletter published by the otherwise quite sensible American Physical Society), rather surprisingly published a new paper by Monckton that tries again to show using rigorous arithmetic that IPCC is all wrong and that climate sensitivity is negligible. His latest sally, like his previous attempt, is full of the usual obfuscating sleight of hand, but to save people the time in working it out themselves, here are a few highlights.

As Deltoid quickly noticed the most egregious error is a completely arbitrary reduction (by 66%) of the radiative forcing due to CO2. He amusingly justifies this with reference to tropical troposphere temperatures – neglecting of course that temperatures change in response to forcing and are not the forcing itself. And of course, he ignores the evidence that the temperature changes are in fact rather uncertain, and may well be much more in accord with the models than he thinks.

But back to his main error: Forcing due to CO2 can be calculated very accurately using line-by-line radiative transfer codes (see Myhre et al 2001; Collins et al 2006). It is normally done for a few standard atmospheric profiles and those results weighted to produce a global mean estimate of 3.7 W/m2 – given the variations in atmospheric composition (clouds, water vapour etc.) uncertainties are about 10% (or 0.4 W/m2) (the spatial pattern can be seen here). There is no way that it is appropriate to arbitrarily divide it by three.

There is a good analogy to gas mileage. The gallon of gasoline is equivalent to the forcing, the miles you can go on a gallon is the response (i.e. temperature), and thus the miles per gallon is analogous to the climate sensitivity. Thinking that forcing should be changed because of your perception of the temperature change is equivalent to deciding after the fact that you only put in third of a gallon because you ran out of gas earlier than you expected. The appropriate response would be to think about the miles per gallon – but you’d need to be sure that you measured the miles travelled accurately (a very big issue for the tropical troposphere).

But Monckton is not satisfied with just a factor of three reduction in sensitivity. So he makes another dodgy claim. Note that Monckton starts off using the IPCC definition of climate sensitivity as the forcing associated with a concentration of 2xCO2 – this is the classical “Charney Sensitivity” and does not include feedbacks associated with carbon cycle, vegetation or ice-sheet change. Think of it this way – if humans raise CO2 levels to 560 ppm from 280 ppm through our emissions, and then as the climate warms the carbon cycle starts adding even more CO2 to the atmosphere, then the final CO2 will be higher and the temperature will end up higher than standard sensitivity would predict, but you are no longer dealing with the sensitivity to 2xCO2. Thus the classical climate sensitivity does not include any carbon cycle feedback term. But Monckton puts one in anyway.